Water-borne diseases are caused by water that has been contaminated by human or animal wastes, and include diseases such as cholera, typhoid, shigella, polio, meningitis, and hepatitis A and E.

Humans can act as hosts to the bacterial, viral, or protozoal organisms that cause these diseases. In many countries where sewage treatment is inadequate, human wastes are disposed of in open latrines, ditches, and canals or are spread on cropland, resulting in extensive diarrheal disease. It is estimated that 4 billion cases of diarrheal disease occur every year, causing 3 million to 4 million deaths, mostly among children. Worldwide, the lack of sanitary waste disposal and of clean water for drinking, cooking, and washing is to blame for over 12 million deaths a year.